MRI antenna

ABSTRACT

In one example, an antenna is disclosed comprising a first, outer conductor, at least one inner conductor within the first outer conductor, and a second outer conductor surrounding the first outer conductor. The second outer conductor may define a plurality of holes therethrough. The at least one inner conductor may comprise a plurality of inner conductors. At least two of the plurality of inner conductors may be connected to each other in series, across a capacitor. At least two of the plurality of inner conductors may be connected to each other in parallel, across a capacitor. The at least one inner conductor may be sufficiently surrounded by the first outer conductor to shield the inner conductor from receiving a radiofrequency signal.

The present application is a division of application Ser. No. 10/996,575, which was filed on Nov. 24, 2004, is assigned to the assignee of the present invention and is incorporated by reference herein. Application Ser. No. 10/996,575 is a continuation of application Ser. No. 09/738,235, which was filed on Dec. 15, 2000, and issued on Jan. 25, 2005 bearing U.S. Pat. No. 6,847,210, which is also assigned to the assignee of the present invention and is incorporated by reference herein. Application Ser. No. 09/738,235 claims the benefit of Application No. 60/172,199, which was filed on Dec. 17, 1999 and is assigned to the assignee of the present invention.

FIELD OF THE INVENTION

This invention relates to radio frequency receiving and transmitting antennas, and, more particularly, to receiving and transmitting radio frequency antennas for use in magnetic resonance imaging.

BACKGROUND OF THE INVENTION

Magnetic resonance imaging (“MRI”) is a well known, highly useful technique for diagnosing abnormalities in biological tissue. MRI can detect abnormalities which are difficult or impossible to detect by other techniques, without the use of x-rays or invasive procedures.

MRI uses changes in the angular momentum or spin of the atomic nuclei of certain elements within body tissue in a static magnetic field after excitation by radio frequency energy, to derive images containing useful information concerning the condition of the tissue. During an MRI procedure, the patient is inserted into an imaging volume containing a static magnetic field. The vector of the angular momentum or spin of nuclei containing an odd number of protons or neutrons tends to align with the direction of the magnetic field. A transmitting antenna within the imaging volume emits a pulse or pulses of radio frequency energy having a particular bandwidth of frequency, referred to as the resonant or Larmor frequency, shifting the vectors of the nuclei out of alignment with the applied magnetic field. The spins of the nuclei then turn or “precess” around the direction of the applied primary magnetic field. As their spins precess, the nuclei emit small radio frequency signals, referred to as magnetic resonance (“MR”) signals, at the resonant or Larmor frequency, which are detected by a radio frequency receiving antenna tuned to that frequency. The receiving antenna is typically positioned within the imaging volume proximate the patient. Gradient magnetic fields are provided to spatially encode the MR signals emitted by the nuclei. After the cessation of the application of radio frequency waves, the precessing spins gradually drift out of phase with one another, back into alignment with the direction of the applied magnetic field. This causes the MR signals emitted by the nuclei to decay. The MR signals detected by the receiving antenna are amplified, digitized and processed by the MRI system. The same antenna may act as the transmitting and receiving antenna. Hydrogen, nitrogen-14, phosphorous-31, carbon-13 and sodium-23 are typical nuclei detected by MRI. Hydrogen is most commonly detected because it is the most abundant nuclei in the human body and emits the strongest MR signal.

The rate of decay of the MR signals varies for different types of tissue, including injured or diseased tissue, such as cancerous tissue. By known mathematical techniques involving correlation of the gradient magnetic fields and the particular frequency of the radio frequency waves applied at various times with the rate of decay of the MR signals emitted by the patient, it is possible to determine the concentrations and the condition of the environment of the nuclei of interest at various locations within the patient's body. This information is typically displayed as an image with varying intensities, which are a function of the concentration and environment of the nuclei of interest. Typical MRI systems are the Quad 7000 and Quad 12000 available from FONAR Corporation, Melville, N.Y., for example.

The quality of the magnetic resonance image is directly related to the characteristics of the receiving and transmitting antenna. Significant electrical characteristics of the antenna include its sensitivity, Q factor and the signal-to-noise ratio.

Sensitivity is the signal voltage generated in the receiving antenna by MR signals of a particular magnitude. The higher the sensitivity within the region to be imaged, the weaker the signals which can be detected. The sensitivity of the antenna is preferably substantially uniform with respect to MR signals emanating from all volume elements within the region of the subject which is to be imaged.

The Q or quality factor, which is closely related to the sensitivity of the antenna, is a measure of the ability of the antenna to amplify the received signal. The Q-value of the antenna can be lowered by a patient proximate or within an antenna, due to capacitive and to a lessor extent the inductive coupling between the patient and the antenna. Antennas must therefore have a high Q-value when they are unloaded and the Q-value must not become too diminished by the presence of the patient. On the other hand, the coil must couple well with the region of a patient's anatomy which is to be imaged.

Signal-to-noise (“S/N”) ratio is the ratio between those components in the electrical impulses appearing at the antenna terminals representing the detected MR signals and the components representing spurious electromagnetic signals in the environment and internally generated thermal noise from the patient. To optimize the S/N ratio, the antenna should have low sensitivity to signals from outside the region to be imaged and to thermal noise. To enhance both S/N ratio and sensitivity, the antenna is “tuned” or arranged to resonate electrically at the frequency of the MR signals to be received (the Larmor frequency), which is typically several megahertz or more. Neither the coil size nor geometry of the antenna can be allowed to create an inductance or self-capacitance which prevents tuning to the desired frequency.

The antenna must also meet certain physical requirements. The antenna should have a high filling factor, which maximizes the amount of tissue which fits within the volume detected by the windings of the coil. The antenna must also fit within the relatively small imaging volumes typically provided for receiving the subject within the magnet assembly, along with other components of the system and the subject. The antenna should not cause significant discomfort to the subject. Additionally, the antenna should be easy to position with respect to the subject, and be relatively insensitive to minor faults in positioning relative to the subject.

These numerous considerations often conflict with one another and therefore must be balanced during the design process.

The sensitivity and S/N ratio of MRI radio frequency receiving antennas have been improved by positioning a first coil, tuned to resonate at the Larmor frequency of the species of interest, proximate the part of the subject which is to be imaged, and positioning a similarly tuned second coil, typically a single loop, adjacent to the first coil. The second coil is connected to the preamplifier of the MRI system. The first and second coils are inductively coupled to each other. MR signals emitted by the patient induce voltages in the first winding, causing current to flow within the winding. The current generates a magnetic field which induces voltage in the second winding. The MR signals may induce voltages in the second coil, as well. The voltages induced in the second coil are processed by the MRI system. Use of such first and second coils amplify the MR signals, and the second coil filters spurious signals outside of the frequency band of the Larmor frequency. See, for example, U.S. Pat. No. 5,583,438 and U.S. Pat. No. 5,575,287, assigned to the assignee of the present invention.

Radio frequency antenna coils may be used in a variety of configurations. For example, the coil may be receiving coil, as discussed above. The receiving coil may be part of an array of receiving coils, such as in the primary and secondary coil arrangements, discussed above. The receiving coil may also act as the transmitting coil of the MRI system. A pair of receiving coils can also be arranged 900 with respect to each other to enable quadrature detection, which improve the signal-to-noise ratio.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the invention, an antenna is disclosed comprising a first, outer conductor, at least one inner conductor within the first outer conductor, and a second outer conductor surrounding the first outer conductor. The second outer conductor may define a plurality of holes therethrough. The at least one inner conductor may comprise a plurality of inner conductors. At least two of the plurality of inner conductors may be connected to each other in series, across a capacitor. At least two of the plurality of inner conductors may be connected to each other in parallel, across a capacitor. The at least one inner conductor may be sufficiently surrounded by the first outer conductor to shield the inner conductor from receiving a radiofrequency signal.

In accordance with another embodiment if the invention, an antenna is disclosed comprising a first conductor and a plurality of second conductors substantially encased by the first conductor. The plurality of second conductors may be sufficiently encased by the first conductor to shield the second conductors from receiving a radiofrequency signal. A third conductor may be provided substantially encasing the first conductor. The third conductor may define a plurality of holes therethrough. At least two of the plurality of inner conductors may be connected to each other in series and at least two of the plurality of inner conductors may be connected to each other in parallel.

In accordance with another embodiment of the invention, an antenna is disclosed comprising a first coaxial cable unit lying in a first plane and a second coaxial cable unit lying in a second plane perpendicular to the first plane. The first coaxial cable unit may comprise a first, inner conductor and a second, outer conductor substantially surrounding the first inner conductor. The second coaxial cable unit may comprise a third, inner conductor and a fourth, outer conductor, substantially surrounding the third inner conductor. The first coaxial cable unit may define a region comprising a perpendicular projection of the first coaxial cable unit and the second coaxial cable unit may lie within the region.

In accordance with another embodiment of the invention, an antenna is disclosed comprising a first coaxial cable unit comprising a first, inner conductor and a second, outer conductor substantially surrounding the first inner conductor. The first and second conductors lie in a first plane. A second coaxial cable unit is provided comprising a third, inner conductor and a fourth, outer conductor, substantially surrounding the third inner conductor. The third and fourth conductors lie in a second plane different than the first plane.

The antennas may be used as receiving and/or transmitting antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a basic coaxial cable unit of an MRI antenna in accordance with one embodiment of the present invention;

FIG. 2 is a cross-sectional view of the basic coaxial cable unit 10, along line 2-2 in FIG. 1;

FIG. 3 shows the basic coaxial cable unit of FIG. 1, with electrical connections to form an antenna in accordance with one embodiment of the present invention;

FIG. 4 is a schematic diagram of a circuit corresponding to the antenna of FIG. 3;

FIG. 5 shows the basic unit of FIG. 1, with a shielding adapter;

FIG. 6 shows another shielding technique;

FIG. 7 shows a variation of the basic unit of FIG. 1, comprising two sections of coaxial cable, and their electrical connections;

FIG. 8 is a schematic diagram of a circuit corresponding to the configuration of FIG. 7;

FIG. 9 is a schematic representation of an antenna array 100 appropriate for imaging of a hand, wrist or toe, using three basic coaxial cable units, in accordance with the present invention;

FIG. 10 is a schematic representation of the side view of the antenna array of FIG. 9;

FIG. 11 is a schematic representation of a circuit corresponding to the antenna array of FIG. 9;

FIG. 12 is a perspective view of the antenna array of FIG. 9, encased in a base of dielectric material, in use with a patient;

FIG. 13 is a schematic diagram of an antenna array in accordance with another embodiment the present invention, which is particularly suited for imaging the head and neck;

FIG. 14 is a side view of the antenna array of FIG. 13, wherein four coaxial units are each in respective vertical, parallel planes, and the center of each unit lies along the same axis;

FIG. 15 is a schematic diagram of a circuit corresponding to the antenna array of FIG. 11;

FIGS. 16-18 show coaxial cable units in various test configurations;

FIG. 19 shows a test configuration of a coaxial cable unit of the present invention;

FIG. 20 shows an antenna array comprising two concentric coaxial cable units lying in the same plane;

FIG. 21 is a schematic diagram of a circuit corresponding to the configuration of FIG. 20;

FIG. 22 shows a quadrature antenna system comprising the hand, wrist and toe antenna array of FIGS. 9-12 and a pair of rectangular coaxial cable units;

FIG. 23 is a top view of the quadrature antenna system of FIG. 22, showing the rectangular coaxial cable units;

FIG. 24 shows a quadrature antenna system of FIG. 23, including additional coaxial cable units coupled to the rectangular coaxial cable units;

FIG. 25 is a side view of the quadrature antenna system of FIG. 24, along line 25-25 in FIG. 24;

FIG. 26 is a side view of the quadrature antenna system of FIG. 24, along line 26-26 of FIG. 25;

FIG. 27 is a schematic diagram of the preamplifier section of an MRI system for use with the quadrature antenna system of FIGS. 22-26;

FIG. 28 is a side view of the gap between the magnet poles of an MRI system;

FIG. 29 is a plan view of a coaxial cable unit in accordance with another embodiment of the invention, including four inner conductors;

FIG. 29 a is a cross-sectional view of the coaxial cable unit of FIG. 29, along line 29-29;

FIG. 30 shows a preferred connection scheme for the inner conductors of the configuration of FIG. 29;

FIG. 31 shows three coaxial cable units as in FIG. 29, separated by an insulator;

FIG. 32 shows the three coaxial cable units of FIG. 31 and preferred connections between their inner and outer conductors;

FIG. 33 is a schematic diagram of a circuit corresponding to the antenna array of FIG. 31;

FIG. 34 shows a triaxial cable unit in accordance with another embodiment of the present invention;

FIG. 34 a is a cross-sectional view of the triaxial cable unit of FIG. 34, along line 34 a-34 a;

FIG. 35 is a schematic representation of the triaxial cable unit of FIG. 34;

FIG. 36 is a schematic representation of the thiaxial cable unit of FIG. 34, wherein the inner conductors are connected in parallel;

FIG. 37 shows a coaxial cable unit as in FIG. 3, wherein the ends of the inner conductor are connected across a capacitor to a radio frequency power source;

FIG. 38 is a schematic representation of the coaxial cable unit of FIG. 37; and

FIG. 39 is a schematic representation of portions of an MRI system, showing in particular a connection between an antenna or antenna array 802 in accordance with the present invention with certain components of the MR1 system;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In examples of embodiments of the present invention, antennas for use in magnetic resource imaging comprise inductively coupled first and second windings tuned to the same frequency, typically the Larmor frequency of species of interest. In a receiving antenna, the second winding is connected to the receiver subsystem of an MRI system. It is believed, without limiting the scope of the invention, that in antennas of embodiments of the present invention, the first winding substantially shields the second winding from direct reception of magnetic resonance (“MR”) signals. However, since the windings are inductively coupled, MR signals detected by the first winding induce voltage signals in the second winding. Since both windings are tuned to the same frequency, highly filtered signals are provided for analysis by the MRI system. The first and second windings may form a coaxial cable. Multiple coaxial cables may be suitably coupled to form antenna arrays for use with different parts of the body.

In a transmitting antenna, the second winding is connected to the radio frequency power source in the radio frequency transmitting section of the MR1 system. Voltage signals provided to the second winding induce voltage signals and current flow in the first winding, causing the emission of highly filtered radio frequency signals.

FIG. 1 is a plan view of a basic coaxial cable unit 10 of an MRI antenna in accordance with one embodiment of the present invention. The unit 10 comprises an inner conductor 12 and an outer conductor 14 coaxially arranged, as shown in the cross-sectional view of FIG. 2. The inner conductor 12 and the outer conductor 14 are separated by a dielectric material 24, such as Teflon®, for example, forming a coaxial cable unit 10. The coaxial conductors 12, 14 are inductively coupled to each other. Preferably, the inner and outer conductors 12, 14 are tightly coupled to each other. More preferably, they are critically coupled to each other.

The inner conductor 12 has two ends 16, 18 and the outer conductor 14 has two ends 20, 22. A plurality of basic units 10 may be connected or coupled to each other in different combinations to form an antenna array, as discussed below.

A body part to be imaged is received in the region 26 bounded by the coaxial cable unit 10. In FIG. 1, the coaxial conductors are in the shape of a ring. Other shapes may be used, dependent on the body part being imaged.

In one example, the outputs 16, 18 of the inner conductor 12 are connected to each other through a capacitor C₁ and the outputs 20, 22 of the outer conductor 14 are connected to each other through a capacitor C₂ to form an antenna 11, as shown in FIG. 3. The capacitors C₁, C₂ have values such that the inner and outer conductors 12, 14 are tuned to the Larmor frequency of the species of interest, such as hydrogen. The capacitor C₁ may be connected to a variable capacitor (“varactor”) in the preamplifier of the receiver system of an MR1 system, or it may be the varactor in the preamplifier. Each conductor 12, 14 may be copper tubing. As shown in FIG. 2, the inner diameter “d₁” of the inner conductor 12 may be about 0.125 inches. The diameter “d₂” of the outer conductor 14 may be about 0.25 inches. The wall thickness of each conductor 12, 14 may be about 0.032 inches.

The coaxial cable unit 10 may be made from readily available soft copper refrigeration tubing of appropriate diameters. Such tubing may be obtained from Metal Product, Wynne, Ark., for example. The tubing corresponding to the inner conductor 12 is covered by a Teflon® tubing, such as TFT70C polytetrafluoroethylene, available from AIN Plastics, Inc., Mount Vernon, N.Y., for example. The Teflon® covered inner conductor 12 is inserted through the tubing corresponding to the outer conductor 14, forming the coaxial cable unit 10. The coaxial cable unit 10 may also be flexible. One-quarter inch, High Power, High Temperature Dielectric Coaxial Cable, Andrew HSTI-50 HELIAX, available from Andrew Corporation, Orland Park, Ill., may be used, for example.

The inner conductor 12 is shielded from direct reception of the MR signals by the outer conductor 12. However, voltage signals induced in the outer conductor 14 induce voltage signals in the inner conductor 12. The outer conductor 14 may, therefore, be modeled as a primary winding of a transformer while the inner conductor 12 may be modeled as the secondary winding of the transformer.

FIG. 4 is a schematic diagram of a circuit corresponding to the antenna of FIG. 3. The inner conductor 12 and the outer conductor 14 are represented as inductors L₁, L₂, respectively. The capacitors C₁ and C₂ are shown, as well. The inductor L₁ and the capacitor C₁ form a first circuit “A” and the inductor L₂ and the capacitor C₂ form a second circuit “B”. The inductors L₁, L₂ are inductively coupled to each other and have a distributed capacitance Cd. The two circuits are tuned to the same, Larmor frequency, forming anti-resonant or parallel resonant, double-tuned circuits. The circuits A, B form a bandpass network which filters frequencies outside of the bandpass of the two circuits. The bandpass of the circuit B (corresponding to the inner conductor 12) is narrower than the bandpass of the circuit A (corresponding to the outer conductor 14), as is known in the art. Noise and other signals outside of the narrower bandwidth of the inner conductor 12 are therefore filtered. As mentioned above, in the embodiment of FIG. 3, the inner conductor 12 is connected to the receiver subsystem of an MRI device, providing highly filtered signals for analysis.

The outer conductor 14 also shields the inner conductor 12 from capacitive coupling with the body of the patient over most of its length. As shown in FIG. 3, however, the inner conductor 12 may not be effectively shielded by the outer conductor 14 in the gap 28 between the ends of the outer conductor 14. To improve the shielding effect of the outer conductor 14 on the inner conductor 12, the ends 20, 22 of the outer conductor 14 may be connected through a copper tube adaptor 30, which closes the gap 28 between the ends of the inner and outer conductors 12, 14, as shown in FIG. 5. The adaptor 30 acts as a capacitive connection between the ends 20, 22 of the outer conductor 14, as well as shielding the outputs 16, 18 of the inner conductor 12. The adaptor 30 also minimizes field distributions in the gap 28 between the ends of the conductors 12, 14.

Two capacitors C₁, C₂ are provided within the adaptor 30, electrically connecting in series the first and second ends 16, 18 of the inner conductor 12 and the first and second ends 20, 22 of the outer conductor 14, respectively. The exposed ends, 16, 18 of the inner conductor 14 and the capacitors C₁, C₂ are shielded by the adaptor 30.

The direct exposure of the inner conductor 12 to MR signals from within the area 28 may also be minimized by distancing the exposed ends 16, 18 from the source of the MR signals in the region 28 and positioning more of the outer conductor 14 between the inner conductor 12 and the source of MR signals within the coil, as shown in FIG. 6.

Dependent on the overall length of the coaxial ring and the magnetic field strength of the magnet used in the MR1 system, in order to tune the antenna 11 of FIG. 3 to the Larmor frequency, the inductance of the inner and outer conductors 12, 14 of the coaxial cable may need to be decreased. The inductance may be decreased by decreasing the lengths of the inner and outer conductors 12, 14, as is known in the art. For example, the inner and outer conductors 12, 14 may be split into two sections 34, 36, as shown in FIG. 7, to facilitate their being tuned to the higher frequencies used in higher field strength magnets. The ends of the inner conductor 12 of a first section 34 are connected in series through respective capacitors C₁, C₃ to the ends of the inner conductor 12″ of the second section 36. The ends of the outer conductor 14″ of the first section 34 are similarly connected in series to the ends of the outer conductors 14″ of the second section 36 through respective capacitors C₂, C₄. The values of the capacitors C₁, C₂, C₃, C₄ are adjusted to tune the conductors 12′, 12″, 14′, 14″ to the desired frequency. Either the capacitor C₁ or the capacitor C₃ may be connected to the variable capacitor in the preamplifier of the MRI system, or may be the variable capacitor in the preamplifier.

FIG. 8 is a schematic diagram of the circuit corresponding to the configuration of FIG. 7. In this case, the inner conductors 12′, 12″ correspond to the inductors L₁, L₂ which are connected in series through capacitors C₁, C₃. The outer conductors 14′, 14″ correspond to the inductors L₃, L₄, respectively, which are connected in series through the capacitors C₂ and C₄. As above, the outer conductor 14 (L₃, L₄) and the inner conductor 12 (L₁, L₂) are inductively coupled.

FIG. 9 is a schematic diagram of an antenna array 100 appropriate for imaging of a hand, wrist or toe, using three coaxial cable ring units 102, 104, 106 as in FIG. 1, in accordance with one embodiment of the present invention. The coaxial ring units 102, 104, 106 lie in three respective parallel, vertical planes P1, P2, P3, respectively, as shown in FIG. 10. To better accommodate the hand, wrist or toe, such that the outer conductors are close to the body party, each unit has a rectangular shape. The antenna array 100 may be used for imaging other body parts, as well. The shape of each unit may be varied as appropriate to accommodate and maintain suitable proximity to the body part.

Each unit 102, 104, 106 has an inner conductor 12, 12′, 12″, respectively, and an outer conductor 14, 14′, 14″. A first, top end 16 of the inner conductor 12 of the first unit 102 is electrically connected in series to a first, bottom end 16′ of the inner conductor 12′ of the second unit 104 through a capacitor C₁. The second, top end 18′ of the inner conductor 12′ is electrically connected in series to the bottom end 16″ of the inner conductor 12″ of the third unit 106 through a capacitor C₂. The second, bottom end 18 of the inner conductor 12 of the first unit 102 and the second, top end 18″ of the inner conductor 12″ of the third unit 106 provide an output 108 of the antenna array 100. The output 108 may be connected across a capacitor (not shown) which may be connected to or may be the varactor of the preamplifier of the receiver system of the MRI device. The output 108 may be connected to the preamplifier through a BNC connector acting as a capacitive module.

The top end 20 of the outer conductor 14 of the first unit 102 is electrically connected in series to the bottom end 20′ of the outer conductor 14′ of the second unit 104, through a capacitor C₃. The top end 22′ of the outer conductor 14′ is electrically connected in series to the first, bottom end 20″ of the outer conductor 14″ of the third unit 106 through a capacitor C₄. The bottom end 22 of the outer conductor 14 of the first unit 102 and the second, top end 22″ of the third unit 106 are electrically connected in series through the capacitor C₅, which is part of a BNC connector acting as a capacitive module.

The antenna array 100 of FIG. 9 may be represented as three-double tuned transformers connected in series, as shown in FIG. 11. The inductors L₁, L₂, L₃ correspond to the inner conductors 12, 12′, 12″ of the first, second and third units 102, 104, 106, respectively, and the inductors L₄, L₅, L₆ correspond to the outer conductor 14, 14′ and 14″. In one implementation, for use in an MRI system including a magnet with a field strength of from about 6,000 Gauss to about 8,000 Gauss, capacitor C₁=68 picofarads; capacitor C₂=56 picofarads; capacitor C₃=22 picofarads; and capacitor C₄=22 picofarads. The inner and outer conductors 12, 14 were tuned to the Larmar frequency of hydrogen for the particular magnet field strength, which in the range of 6,000 to 8,000 Gauss is between about 25-35 megahertz. The length “D1” (FIG. 9) of each coaxial cable unit 102, 104, 106 was 7 inches. The distance “D₂” (FIG. 10) between the center of adjacent coaxial cable units was 1.5 inches. The Q of the antenna array 100 was found to be about 197 while the S/N ratio was found to be 320 to 1.

While an antenna array comprising three coaxial cable units preferred, the antenna array 100 could comprise the two coaxial cables 102 and 106, in which case, the first end 16 of the inner conductor 12″ of the first coaxial cable 102 would be connected to the first end 16 of the inner conductor 12 coaxial cable unit 106 and the first end 20 of the outer conductor 14 would be connected to the first end 20″ of the outer conductor 14″ coaxial cable unit 106. The outputs of the antenna array would be the same.

The antenna array 100 is preferably encased in a base 100 a of a rigid, dielectric material, such as the fire resistant polymers polyvinyl chloride, polytetrafluoroethylene and fluorinated ethylene propylene, as shown in FIG. 12. Polymers having low dielectric constants are particularly preferred. In FIG. 12, the first, second and third coaxial cable units 102, 104, 106 are shown in phantom. The coaxial cable units 102, 104, 106 are coaxially aligned along an axis A through the center of the base I 00 a, as discussed above with respect to FIG. 11. The base 100 a is connected to a plate 100 b which may be positioned on the patient bed within the gap of the MRI magnet. A patient's hand (“h”) is shown, partially inserted into a region within each of the coaxial cable units 102, 104, 106, for magnetic resonance imaging of the fingers, for example. A strap 100 c may be provided to restrain movement of the hand h during imaging.

Flexible coaxial cable units can also be supported by a flexible belt. An example of a flexible coaxial cable is described above. The flexible belt can comprise cross-linked polyethylene foam, available from Contour Fabricators, Inc., Grand Blanc, Mich.

FIG. 13 is a schematic diagram of an antenna array 200 in accordance with another embodiment the present invention, which is particularly suited for imaging the head and neck. FIG. 14 is a side view of the array, wherein four coaxial ring units 202, 204, 206, 208 are each in respective vertical, parallel planes P1, P2, P3, P4 and the center of each unit lies along the same axis “A”.

Returning to FIG. 13, because larger conductors are needed to surround the head, each coaxial ring unit preferably has two sections to lower the inductance of the conductor, as discussed above with respect to the embodiment of FIG. 7. Each section has an outer conductor with first and second ends and an inner conductor with first and second ends.

In the first unit 202, the upper section 202 a has an inner conductor 210 with first and second ends 210 a, 210 b and an outer conductor 214 with first and second ends 214 a, 214 b. Similarly, the lower section 202 b has an inner conductor 216 with first and second ends 216 a, 216 b and an outer conductor 218 with first and second ends 218 a, 218 b. The first end 210 a of the inner conductor 210 is electrically connected in series to the first end 216 a of the inner conductor 216 through a capacitor C₁. The first end 214 a of the outer conductor 214 is electrically connected in series to the first end 218 a of the outer conductor 218 through a capacitor C₂. The second end 210 b of the inner conductor 210 and the second end 216 b of the inner conductor 216 provide an Output. The Output would be connected to a varactor in the preamplifier of the receiver system of the MRI device, optionally through a capacitor. The second end 214 b of the outer conductor 214 is electrically connected in series to the second end 218 b of the outer conductor 218 through a capacitor C₃ of a BNC connector capacitive module.

In the second unit 204, the upper section 204 a has an inner conductor 220 with first and second ends 220 a, 220 b and an outer conductor 222 with first and second ends 222 a, 222 b. Similarly, the lower section 204 b has an inner conductor 224 with first and second ends 224 a, 224 b and an outer conductor 226 with first and second ends 226 a, 226 b. The first end 220 a of the inner conductor 220 is electrically connected in series to the first end 224 a of the inner conductor 224 through a capacitor C₄. The first end 222 a of the outer conductor 222 is electrically connected in series to the first end 226 a of the outer conductor 226 through a capacitor C₅. The second end 220 b of the inner conductor 220 and the second end 224 b of the inner conductor 224 are electrically connected in series through a capacitor C₁₄. The second end 222 b of the outer conductor 222 is electrically connected in series to the second end 226 b of the outer conductor 226 through a capacitor C₆.

In the third unit 206, the upper section 206 a has an inner conductor 228 with first and second ends 228 a, 228 b and an outer conductor 230 with first and second ends 230 a, 230 b. Similarly, the lower section 206 b has an inner conductor 232 with first and second ends 232 a, 232 b and an outer conductor 234 with first and second ends 234 a, 234 b. In the fourth unit 208, the upper section 208 a has an inner conductor 236 with first and second ends 236 a, 236 b and an outer conductor 238 with first and second ends 238 a, 238 b. The lower section has an inner conductor 240 with first and second ends 240 a, 240 b and an outer conductor 242 with first and second ends 242 a, 242 b.

The first end 228 a of the inner conductor 228 of the third unit 206 is electrically connected in series to the first end 232 a of the inner conductor 232 through a capacitor C₇. The first end 230 a of the outer conductor 230 is electrically connected in series to the first end 234 a of the outer conductor 234 through a capacitor C₈. The second end 230 b of the outer conductor 230 is electrically connected in series to the second end 232 b of the inner conductor 232 through a capacitor C₉. The second end 228 b of the inner conductor 228 is electrically connected in series to the second end 242 b of the outer conductor 242 of the lower section 208 b of the fourth unit 208 through a capacitor C₁₀. The second end 234 b of the outer conductor 234 is electrically connected in series to the second end 236 b of the inner conductor 236 of the upper section 208 a of the fourth unit 208 through a capacitor C₁₁. The second end 238 b of the outer conductor 238 is electrically connected in series to the second end 240 b of the inner conductor 240 through a capacitor C₁₂. The first end 236 b of the inner conductor 236 is electrically connected in series to the first end 240 a of the inner conductor 240 through a capacitor C₁₃. The first end 238 a of the outer conductor 238 is electrically connected in series to the first end 242 a of the outer conductor 242 through a capacitor C₁₄.

In operation, a magnetic field is generated between the third unit 206 and the fourth unit 208. The fourth unit 208, which has a smaller diameter than the third unit 206, reflects the magnetic field towards the third unit 206. The first and second units 202, 204 act as receiver coils and resonators within the magnetic field created by the third and fourth units.

FIG. 15 is a corresponding circuit diagram of the coaxial cable units of FIG. 11. The inductors of FIG. 13 correspond to the inner and outer conductors of FIG. 11, as follows:

L₁ Inner Conductor 210 L₂ Inner Conductor 216 L₃ Outer Conductor 214 L₄ Outer Conductor 218 L₅ Inner Conductor 220 L₆ Inner Conductor 224 L₇ Outer Conductor 222 L₈ Outer Conductor 226 L₉ Inner Conductor 228 L₁₀ Inner Conductor 232 L₁₁ Outer Conductor 230 L₁₂ Outer Conductor 234 L₁₃ Inner Conductor 236 L₁₄ Inner Conductor 240 L₁₅ Outer Conductor 238 L₁₆ Outer Conductor 242

The corresponding capacitors in FIGS. 11 and 13 are commonly identified.

In one configuration of the head and neck antenna array 200 for the use in an MRI system with a magnet with a field strength of between about 6,000 Gauss to about 8,000 Gauss, the capacitors C₁, C₂, C₄, C₉, C₁₁ and C₁₄ had a value of 220 picofarads (“pf”). Capacitor C₃ had a value of 470 pf and Capacitor C₁₀ had a value of 820 pf. The inner and outer conductors are preferably tuned to a frequency between about 25 to about 35 MHz. The first, second and third coaxial cable units 202, 204, 206 had an outer height “h1” (FIG. 13, FIG. 14) of about 11 inches. The fourth coaxial ring unit 208 had an outer diameter “D2” of about 9 inches. The distance “D” between adjacent coaxial cable units was 2 inches. (FIG. 14) Each coaxial cable unit was critically coupled to an adjacent coaxial cable unit. Such an antenna array 200 was found to have a Q of about 220 and a S/N ratio of about 360 to 1.

When positioned over the head of a patient, the third unit 204 is preferably positioned over the eyes of the patient.

The antenna array 200 may be encased in a base 200 a of rigid dielectric material, as discussed above with respect to FIG. 13.

Testing of several of the configurations discussed above show significant improvements in Q and S/N ratio. The Q of the antenna may be measured by a network analyzer, such as the 3577A, available from the Hewlett Packard Company. The S/N ratio may be measured by performing a phantom scan by an MRI system, as is known in the art. In one test, the ends 20, 22 of an outer conductor 14 were electrically connected in series across a variable capacitor C₁, as shown in FIG. 16. The range of the variable capacitor C₁ was from 0 to about 30 picofarads. The outer conductor was tuned to a frequency between 25 to 35 MHz. The Q and S/N ratio of the coaxial cable in FIG. 16 were found to be 59 and 180, respectively. In FIG. 17, the ends 16, 18 of the inner conductor 12 were connected across the variable capacitor C₁, and the ends 20, 22 of the outer conductors were connected across a capacitor C₂ having a value of 10 picofarads. The inner and outer conductors 12, 14 were tuned to the frequency of between about 25 to 35 MHz. The Q of the circuit was 72 and the S/N ratio was 220, approximately a 22% improvement in both characteristics. The use of multiple coaxial cable units has been found to further improve the Q and S/N ratio of the antenna. As mentioned above, the Q of the hand, wrist and toe antenna array 100 of FIG. 10, for example, was found to be about 197, about a 233% improvement. The S/N ratio was found to be about 320, about an 82% improvement.

When only the inner ends 16, 18 of the conductor 12 of a coaxial cable unit were connected across the capacitor C₁, and a capacitor C₃ of 120 picofarads, and the inner conductor was tuned to a frequency between about 25 to 35 MHz, as in FIG. 18, the Q of the circuit 28 was 103. The S/N ratio was 160. In the configuration of FIG. 19, the ends 16, 18 of the inner conductor 12 and the ends 20, 22 of the outer conductor 14 were connected through capacitors C₁, C₄, C₅ and C₆, respectively. The capacitors C₄=72 pf, the capacitor C₅=68 pf and the capacitor C₆=47 pf. Both conductors were tuned to a frequency between about 25 to 35 MHz. The Q of the circuit was 128 and the S/N ratio was 200 to 1, approximately a 25% improvement. As mentioned above, the use of multiple coaxial cables further improves the Q and S/N of the antenna. The Q of the head antenna array 200, with four coaxial cables, was 220, about a 113% improvement over the configuration of FIG. 18. The S/N ratio was 360 to 1, about a 125% improvement.

The basic coaxial cable unit 10 of FIG. 1 may also be used in antennas in other configurations, in accordance with the present invention. For example, in FIG. 20, an antenna array 400 comprises two concentric coaxial cable units 402 and 404, lying in the same plane. The inner coaxial cable unit 402 comprises an inner conductor 406 and an outer conductor 408. The inner conductor 406 has a first end 414 and a second end 418. The outer coaxial cable unit 404 comprises an inner conductor 410 and an outer conductor 412. The inner conductor 410 has a first end 416 and a second end 420. The outer conductor 412 has a first end 422 and a second end 424.

The first end 414 and the second end 418 of the inner conductor 406 of the inner coaxial cable unit 402 are electrically connected in parallel to the first end 416 and the second end 420, respectively, of the inner conductor 410 of the outer coaxial cable 404. A capacitor C₁ is electrically connected in parallel to the inner conductors 406 and 410.

The first end 422 of the outer conductor 412 is electrically connected to the second end 424 of the outer conductor 412, through a capacitor C₂. A portion 408 a of the outer conductor 408 of the inner coaxial cable unit 402 is also directly electrically connected to an adjacent portion 412 a of the outer conductor 412 and a portion 408 b of the outer conductor 408 is directly electrically connected to a portion 412 b of the outer conductor 412 b, through electrical contacts 411, 413, respectively. Insulation (not shown) may be provided between the outer conductors 408 and 412 along the remainder of their lengths.

The inner conductors 406, 410 of the two coaxial cables 402, 404 provide the output of the antenna 400. The capacitor C₁, may be connected to a variable capacitor in the preamplifier of the MRI system, or be the variable capacitor of the preamplifier.

As above, the values of the capacitors C₁, and C₂ are such that the circuit including the inner conductors and the circuit including the outer conductors, are tuned to the same Larmor frequency.

FIG. 21 is an schematic diagram of a circuit corresponding to the configuration of FIG. 20, wherein the inductor L₁, represents the outer conductor 412 of the outer coaxial cable unit 404 and the inductor L₂ represents the outer conductor 408 of the inner coaxial cable unit 402. The inductor L₃ represents the inner conductor of 410 of the outer coaxial cable unit 404 and the inductor L₄ represents the inner conductor 406 of the inner coaxial cable unit 402.

The antennas and antennas arrays of the present invention may also be used in a quadrature arrangement. For example, in FIG. 22, a quadrature antenna system 450 is shown comprising the hand, whist and toe antenna array 100 of FIGS. 9-12 and a pair of rectangular coaxial cable units 452 a, 452 b. FIG. 23 is a top view of the antenna arrays 100 and 452, showing the rectangular coaxial cable unit 452. In FIG. 23, the rectangular coaxial cable coil 452 lies in a flat plane A of the page. In FIG. 22, the flat plane A is perpendicular to the page and is perpendicular to the parallel planes P₁, P₂, P₃, of the antenna array 100.

In FIG. 22, the antenna array 100 and the rectangular or coaxial cable 452 are separated by a dielectric material 454, such as plastic. The dielectric material 454 may be in the form of a plate. The rectangular coaxial cable 452 may also be supported by a plate 456 of plastic or other dielectric material. The plates 502, 503 may be 1 inch thick, for example.

Returning to FIG. 23, the rectangular coaxial cable unit 452 preferably includes two sections 452 a and 452 b to lower the inductance of the coaxial cable 452. The two sections 452 a, 452 b each have an inner conductor 12′, 12″, and an outer conductor 14′, 14″, as in FIG. 7. Opposing ends 16 a, 18 a and 16 b, 18 b of the inner conductors 12′ and 12″ are connected to each other through capacitors C₁, and C₄, while the opposing ends 20 a, 22 a and 20 b, 22 b of the outer conductors 14′ 14″, are connected to each other through capacitors C₂, C₃. The output of the rectangular coaxial cable unit 452 is across the capacitor C₁. As above, the capacitor C₁, may be connected to a varactor in the preamplifier of the MRI system or may be the varactor in the preamplifier.

A capacitor C₅ is connected across the output 108 of the antenna array 100 (see FIG. 9). The details of the connections of the inner and outer conductors of each coaxial cable unit of the array 100, and of the output across the capacitor C₅, are not shown in FIG. 23. As above, the capacitor C₅ may also be connected to or be a varactor in the preamplifier of the MRI system.

The range of the rectangular coaxial cable units 452 a, 452 b may be extended by providing additional coaxial cable units 454, 456, perpendicular to the coaxial cable units 452 a, 452 b. FIG. 24 shows a quadrature antenna system 450′ of FIG. 23, which includes additional coaxial cable units 454, 456. The coaxial cable unit 454 includes an inner conductor 458 and an outer conductor 460. The coaxial cable unit 456 includes an inner conductor 462 and an outer conductor 464. FIG. 25 is a side view of the quadrature antenna system 450, along line 25 in FIG. 26. FIG. 26 Is a side view of the quadrature antenna system 450, along arrow 26 in FIG. 26. The height “h” of the coaxial cable units 454, 456 may be 4 inches, for example. The antenna array 100 is not shown in FIGS. 25 and 26.

As shown in FIGS. 24 and 25, a first end 460 a of the outer conductor 460 of the coaxial cable unit 454 is connected to the second end 22 b of the inner conductor 12″ of the coaxial cable unit 452 b through a capacitor C₁₀. The first end 458 a of the inner conductor 458 is directly connected to the first end 18 b of the outer conductor 14″.

The second end 460 b of the outer conductor 460 is directly connected to the second end 20 b of the coaxial cable unit 452 a. The second end 16 b of the inner conductor 12′ of the coaxial cable unit 452 a and the second end 458 b of the inner conductor 458 of the coaxial cable unit 454 are connected across a capacitor C₁₁.

As shown in FIGS. 24 and 26, a first end 462 a of the inner conductor 462 of the coaxial cable unit 456 is connected to the first end 18 a of the inner conductor 12″ of the coaxial cable unit 452 b through a capacitor C₁₂. The second output of the quadrature antenna system is provided across the capacitor C₁₂. The first end 464 of the outer conductor 464 a is connected to the second end 22 a of the outer conductor 14″ through a capacitor C₁₄.

The second end 462 b of the inner conductor 462 is directly electrically connected to the first end 16 a of the inner conductor 12′ of the coaxial cable unit 452 a. The second end 464 b of the outer conductor 464 is directly connected to the first end 20 a of the outer conductor 14′.

The connections within the antenna array 100 are the same as in FIG. 23 and FIG. 9.

FIG. 27 is a schematic diagram of the preamplifier section 470 of an MRI system for use with the quadrature antenna systems 100 and 452. Two preamplifiers; 472, 474 are shown, each providing an input 476, 478 to a phase shifter signal combiner 480, which provides an output 482 to the signal processing portion of an MRI system. The output from the antenna array 100 may be provided to the preamplifier 472 and the output from the rectangular coaxial cable antenna 452 may be provided to the preamplifier 472 for example. The phase shifter may be a Mini-Circuit 15542 ZSCQ-2-50, available from Mini-Circuit, Brooklyn, N.Y., for example.

FIG. 28 is a side view of the gap 490 between the magnet poles 492, 494 of an MRI system, such as the Quad 7000 available from FONAR, Corporation, Melville, N.Y. The quadrature antenna system 450 rests on a patient bed 496 portion of which is above the pole 494. Preferably, the quadrature antenna system 450 is supported at an angle with respect to the patient bed 496. An angle of about 15° may be provided, for example. The quadrature antenna system 450 may be supported at an angle by a wedge 498, for example.

The head and neck antenna array 200 may also be used in a quadrature arrangement by adding a coaxial cable unit perpendicular to the planes of the coaxial cable units of the antenna array 200, above or below the antenna array.

It is known that the voltage induced in a secondary winding of a transformer may be increased by increasing the number of turns in the secondary winding. The voltage induced in the inner conductor of the coaxial cable unit 10 of FIG. 1 may therefore be increased by connecting a plurality of inner conductors in series. The signal-to-noise (S/N) ratio is thereby improved. However, it is also known that connecting inductors in series increases the resistance of the circuit, decreasing the Q of the circuit. In addition, the inductance is also increased, preventing tuning of the circuit to high frequencies for use with magnets of high field strengths, as discussed above. Therefore, it is preferred to also provide inner conductors connected in parallel, to lower the resistance and inductance of the circuit including the inner conductors. Parallel connection also increases current flow, which also improves the signal-to-noise (S/N) ratio.

FIG. 29 is a plan view of a coaxial cable unit 500 in accordance with another embodiment of the invention, including multiple inner conductors. In the configuration of FIG. 29, four inner conductors 504, 506, 508, 510 are shown. More or fewer inner conductors may be provided. FIG. 29 a is a cross-sectional view of the coaxial unit 500 along line 29 a-29 a in FIG. 29. Preferably, the inner conductors 504, 506, 508, 510 are closely and tightly bundled by slightly twisting them together. The inner conductors may be solid and are covered by a dielectric material, such as Teflon tubing (not shown).

The ends 502 a, 502 b of the outer conductor are connected across a capacitor C₁, as shown in FIG. 29.

A preferred connection scheme for the inner conductors is shown in FIG. 30, where the inductors L₂, L₃, L₄ and L₅ correspond to the inner conductors, 504, 506, 508, 510, respectively. The inductors L₂ and L₃ are preferably connected in series through capacitor C₂. The inductor L₃ and the inductor L₄ are also preferably connected in series, through a capacitor C₃. The inductors L₄ and L₅ are preferably connected in parallel. The parallel connector inductors L₄, L₅ are electrically connected to the inductor L₃ though a capacitor C₃. The resistance and inductance are not, therefore, increased as much as if all of the inductors were connected in series.

The circuit including the outer conductor and the circuit including the inner conductor are tuned to the same Larmor frequency, as discussed, above. As above, the capacitor C₄ may be connected to a varactor in the preamplifier of the MRI system or may be the varactor.

An antenna array may also be provided with multiple coaxial cable units 500. FIG. 31 shows three coaxial cable units 520, 522 and 524, separated by an insulator 526 such as Teflon®. Each coaxial cable 520, 522, 524, may be copper tubing with a diameter of about 0.25 inches. The Teflon® may be 1 mm thick, for example. FIG. 33 shows the three coaxial cable units 520, 522, 524 and the connections between their inner and outer conductors. Those connections will be explained with respect to the electrical schematic diagram of FIG. 33, wherein the top box corresponds to the coaxial cable unit 520, the middle box corresponds to the coaxial cable unit 522 and the lower box corresponds to the coaxial cable unit 524.

The outer conductors of each coaxial cable unit 520, 522, 524 are represented as inductors L₁, L₂ and L₃, respectively, in FIG. 33. The inductors L₁, L₂ and L₃ are preferably connected in parallel across a capacitor C₁. The parallel connection of the outer conductors provides better shielding of the inner conductors. Alternatively, the outer conductors may be connected in series.

In the coaxial cable unit 520, two of the inner conductors are preferably connected in series and two are preferably connected in parallel. In FIG. 33, the inductors L₄, L₅, L₆ and L₇ represent the four inner conductors of the first coaxial cable unit 520. The two inductors L₄ and L₅ are connected in series across a capacitor C₂. The two inductors L₆ and L₇ are connected in parallel. The parallel connected inductors L₆, L₇ are connected to the inductor L₅ through a capacitor C₃.

In the second coaxial cable unit 522, two pairs of the inner conductors are connected in parallel. In FIG. 33, the inner conductors are represented by the inductors L₈, L₉, L₁₀ and L₁₁, respectively. The inductors L₈ and L₉ are connected in parallel and the inductors L₁₀ and L₁₁ are connected in parallel. One pair (L₈, L₉) is connected to the other pair (L₁₀, L₁₁) across a capacitor C₄. The pair of parallel connected inductors (L₆, L₇) in the first coaxial cable unit is also connected to one of the pairs of parallel connected inductors (L₈, L₉) in the second coaxial cable unit 522 through a capacitor C₅.

In the third coaxial cable unit 524, three of the inner conductors are preferably connected in parallel. In FIG. 33, the inner conductors are represented as the inductors L₁₂, L₁₃, L₁₄, and L₁₅, respectively. The inductor L₁₂ is connected to the three parallel connected inductors L₁₃, L₁₄, L₁₅ through a capacitor C₆. While the capacitor C₆ is shown within the box 524 representing the third coaxial cable unit 524, it is most readily provided between the second and third coaxial cable units 522, 524, as shown in FIG. 32. One of the pairs of parallel connected inductors (L₁₀, L₁₁) in the second coaxial cable unit 522 is also connected in parallel to a first one of the inductors L₁₂ in the third coaxial cable unit 524.

One end the first inductor L4 of the first coaxial cable unit 520 and one end of the three parallel connected inductors L₁₃, L₁₄, L₁₅ in the third coaxial cable unit 524 are connected across a capacitor C₇ to provide an output of the antenna array. As discussed above, the capacitor C₇ may be connected to a varactor in the preamplifier of the MRI system or may be the varactor in the preamplifier.

As above, the circuit of the connected inner conductors is tuned to the same frequency as the circuit including the outer conductors. Such a configuration was found to have a signal-to-noise (S/N) ratio of about 700 and a Q of about 200.

The outer conductor 504 of the coaxial cable unit 502 may have an inner diameter “D1” of 0.25 inches, as shown in FIG. 29 a. The outer diameter “D2” (FIG. 29) of the coaxial cable unit ring may be 6¾ inches, for example. Such a coaxial cable unit is appropriate for imaging a knee, for example. The inner conductors 504, 506, 508 and 510 may each have an outer diameter of 0.74 millimeters. As stated above, the inner conductors 504, 506, 508, 510 are preferably tightly bundled.

For coaxial cable units with diameters D1 (FIG. 29) of greater than about 8½ inches, or for use with high magnetic field strengths and higher Larmor frequencies, it is preferred to connect all of the inner conductors in parallel, to facilitate tuning. A parallel connection scheme is discussed below with respect to FIG. 36.

In another embodiment, an additional outer conductor may be added to the coaxial cable unit 500 of FIG. 29, forming a triaxial cable unit 600, as shown in FIG. 34. Two outer conductors 602, 604 are provided with multiple inner conductors 606, 608, 610, 612. The outermost, first outer conductor 602, which surrounds the second outer conductor 604, has openings 614 distributed around its periphery. FIG. 34 a is a cross-sectional view of the triaxial cable unit 600 along line 34 a-34 a of FIG. 34, showing the various components.

Ends of the first outer conductor 602 are connected through a capacitor C₂. Ends of the second outer conductor 604 are also connected to each other through a capacitor C₁. The first and second outer conductors 602, 604 are both tuned to the Larmor frequency of the species of interest. Depending on the diameter D of the ring 600, and/or the magnetic field strength, the inner conductors 606, 608, 610, 612 are connected to each other as described above with respect to FIG. 30, or in parallel, as discussed below.

FIG. 35 is a schematic representation of a circuit corresponding to the triaxial cable unit of FIG. 34, wherein the first outer conductor 602 is represented by the inductor L₁, the second outer conductor 604 is represented as the inductor L₂ and the inner conductors 606, 608, 610, 612 are represented as inductors L₃, L₄, L₅, L₆, respectively. As discussed above, ends of the outer conductors 602, 604 (L₁, L₂) are connected across the capacitors C₁, C₂, respectively. Two of the inner conductors 606, 608 (L₃, L₄) are connected in series and the other two inner conductors 610, 612 (L₅, L₆) are connected in parallel, as shown in FIG. 35.

FIG. 36 is a schematic representation of the triaxial cable unit 600 of FIG. 34, wherein the inner conductors 606, 608, 610, 612 (L₃, L₄, L₅, L₆ are connected in parallel.

In one configuration, the triaxial cable unit ring has an inner diameter “D” of 8 inches. The inner diameter “d1” (FIG. 34 a) of the first conductor is ⅜ inch. The inner diameter “d2” of the second conductor is ¼ inch. The holes 614 in the first conductor 602 have diameters of about 2 mm. and are separated by a length “L” of about 1 inch.

Because of the holes 614 through the first, outermost conduct 602, it is believed, without limiting the scope of the invention, that magnetic resonance signals are detected by both the first and second outer conductors 602 and 604, and that in addition, since the first and second outer conductors 602, 604 are inductively coupled, voltage signals which are a function of the magnetic resonance signals detected by the first outer conductor 602, are induced in the second outer conductor 604. The second outer conductor is also inductively coupled to the inner conductors 606, 608, 610, 612 and induces voltage signals in the inner conductors which are provided to the MRI system for processing. While the embodiment of FIGS. 34-36 has multiple inner conductors 504, 506, 508, 510, a second outer conductor 604 may be used in a triaxial cable with only one inner conductor.

As mentioned above, the coaxial cable units of the present invention may also be used as transmitting antennas. FIG. 37 shows a coaxial cable unit 700 as in FIG. 3, wherein the ends 16, 18 of the inner conductor 12 are connected across a capacitor C₁, to a radio frequency power source. The ends 20, 22 of the outer conductor 14 are connected across a capacitor C₂. The inductance of the inner and outer conductors 12, 14 and the values of the capacitors are adjusted so that the respective circuits are tuned to substantially the same frequency.

FIG. 38 is a schematic representation of the coaxial cable unit of FIG. 37, where the inner conductor 12 is represented by the inductor L₁, and the outer conductor 14 is represented by L₂. In this case, a time varying voltage in the inner conductor 12 (L₁) induces a time-varying voltage in the outer conductor 14 (L₁), causing emission of radio frequency pulses from the outer conductor 14 (L₁). The first circuit A comprising the inductor L₁, and the capacitor C₁, filters the driving signal from the RF power source to within a first bandwidth. The second circuit B comprising the inductor L₂ and the capacitor C₂ filters the signal received from the first circuit A to within a narrower bandwidth. The signal transmitted by the circuit B therefore includes less noise than conventional transmitting antennas.

Any of the coaxial cable configurations discussed above can act as both a receiving and transmitting antenna.

FIG. 39 is a schematic representation of portions of an MRI system 800, showing in particular a connection between an antenna or antenna array 802 in accordance with the present invention with certain components of the MRI system. A signal processing system 804, a computer 806, an NMR controller 808, a gradient coil subsystem 810, a receiver subsystem 812 including a preamplifier 116, and an image display system 114 of the MRI system 800 are also shown. The computer 806 controls the overall operation of the MRI system 800. The NMR controller 808 stores the timing of the scanning sequences and controls implementation of the scanning sequence. The signal processing system 804 typically includes a variable amplifier, a frequency down converter, an analog-to-digital converting array and a digital data processor (not shown), as is known in the art. The gradient coil subsystem 810 includes the gradient coils and a gradient waveform generator, which outputs particular waveforms for a desired scanning sequence to the gradient coils under the control of the NMR controller 808, also as is known in the art. The particular ends of the inner conductor of the coaxial cable unit providing the output of the antenna array 802 are connected to the receiver system 812 of the MRI system 800 through the port 818.

As discussed above, the presence of a patient provides a load on the antenna array 802 which lowers the antenna's Q. The presence of the patient also shifts the resonant frequency of the antenna array 802, which may require returning to the desired Larmor frequency. A varactor, or vanable capacitor 820 is therefore provided between the capacitor C₁ of the antenna 802 and the preamplifier 816 parallel to the capacitor C₁, to enable returning of the antenna array 802 when the antenna array is positioned with respect to the patient, as is known in the art. Alternatively, C₁, may be the varactor 520. A back diode 822 is preferably provided parallel to the varactor 820 to prevent the passage of excessive voltage to the preamplifier 816, also as is known in the art. Voltage greater than about 0.7 volts is typically blocked by the back diode 822. The varactor 820 is controlled by the computer 806. The port 818 may be connected to the varactor 820, back diode 552 and preamplifier 810 through a short, low capacitance cable, or other appropriate means.

FIG. 39 also shows an optional connection between the capacitor C₆ and the antenna 802, and the RF transmitting subsystem 824 of the MRI system, through the optional port 826. The RF transmitting subsystem includes an RF power source (not shown) for driving the transmitting antenna, as is known in the art. The RF transmitting subsystem 824 can also be connected to the capacitor C₁, through a switch controlled by the computer 806. The computer 806 would switch the connection between the RF subsystem 824 and the antenna 802, and the preamplifier 820 and the antenna 802, at appropriate times. The RF transmitting subsystem 824 is controlled by the NMR controller 808 and the computer 806 of the MRI system 800.

A more complete description of the structure and operation of the MRI system may be found in U.S. Pat. No. 6,025,717, assigned to the assignee of the present invention and incorporated by reference, herein.

The above embodiments are examples of antennas and magnetic resonance imaging systems in accordance with the present invention. It will be recognized by those skilled in the art that changes may be introduced to those embodiments without going beyond the scope of the invention, which is defined by the claims, below. 

1. An antenna comprising: a first, outer conductor; at least one inner conductor within the first outer conductor; the at least one inner conductor being inductively coupled to the first outer conductor and electrically unconnected to the first outer conductor; and a second outer conductor surrounding the first outer conductor; wherein: the second outer conductor defines a plurality of holes therethrough; and the antenna forms a loop configured to receive a body part.
 2. The antenna of claim 1, wherein: the at least one inner conductor has respective ends connectable across at least one first capacitor to tune the at least one inner conductor to a frequency; the first outer conductor has ends connectable across at least one second capacitor to tune the first outer conductor to the frequency; the second outer conductor has ends connectable across at least one third capacitor to tune the second outer conductor to the frequency; the at least one inner conductor is inductively coupled to the first outer conductor; and the first outer conductor is inductively coupled to the second outer conductor.
 3. The antenna of claim 1, wherein an output of the antenna is provided from the at least one inner conductor.
 4. The antenna of claim 1, wherein: the at least one inner conductor comprises a plurality of inner conductors.
 5. The antenna of claim 4, wherein: at least two of the plurality of inner conductors are connected to each other in series, across a capacitor.
 6. The antenna of claim 4, wherein: at least two of the plurality of inner conductors are connected to each other in parallel, across a capacitor.
 7. The antenna of claim 1, wherein the at least one inner conductor is sufficiently surrounded by the first outer conductor to shield the inner conductor from receiving a radiofrequency signal.
 8. The antenna of claim 1, wherein the first outer conductor, the inner conductor, and the second outer conductor form a first triaxial cable unit, the antenna further comprising: a second triaxial cable unit, adjacent to the first triaxial cable unit, the second triaxial cable unit comprising: a fourth outer conductor, at least one fifth inner conductor within the fourth outer conductor, and a sixth outer conductor surrounding the fourth outer conductor; wherein the fifth inner conductor and the second inner conductor are electrically coupled.
 9. The antenna of claim 8, wherein the fifth inner conductor and the second inner conductor are electrically coupled across a capacitor.
 10. The antenna of claim 8, wherein the fifth inner conductor and the second inner conductor are directly electrically connected.
 11. The antenna of claim 1, wherein the first outer conductor is inductively coupled to the second outer conductor and the first outer conductor is electrically unconnected to the second outer conductor.
 12. The antenna of claim 1, wherein: the antenna is a receiving antenna; the second outer conductor is configured to receive electromagnetic radiation; the first outer conductor is configured to receive some electromagnetic radiation directly, though the plurality of holes; and the second outer conductor is electrically unconnected to the first outer conductor and the second outer conductor is inductively coupled to the first inner conductor.
 13. An antenna comprising: a first conductor; and a plurality of second conductors surrounded by the first conductor; wherein: the first conductor is inductively coupled to the plurality of second conductors, during operation; the plurality of second conductors are electrically unconnected to the first conductor; at least some of the plurality of second conductors have respective ends connectable across at least one respective capacitor to tune the inner conductors to a frequency; and the first conductor has ends connectable across at least one capacitor to tune the first conductor to the frequency.
 14. The antenna of claim 13, further comprising: a third conductor surrounding the first conductor; wherein the third conductor defines a plurality of holes therethrough.
 15. The antenna of claim 13, wherein: at least two of the plurality of second conductors are connected to each other in series; and at least two of the plurality of second conductors are connected to each other in parallel, across a capacitor.
 16. The antenna of claim 13, wherein an output of the antenna is provided from the at least one of the plurality of second conductors.
 17. The antenna of claim 13, wherein at least two of the plurality of second conductors are connected to each other in parallel, across at least one capacitor.
 18. The antenna of claim 13, wherein at least two of the plurality of second conductors are connected to each other in series, across at least one capacitor.
 19. An antenna comprising: a first, outer conductor; at least one inner conductor within the first outer conductor; the at least one inner conductor being inductively coupled to the first outer conductor and electrically unconnected to the first outer conductor; and a second outer conductor surrounding the first outer conductor; wherein the second outer conductor defines a plurality of holes therethrough; the at least one inner conductor comprises a plurality of inner conductors; and at least two of the plurality of inner conductors are connected to each other in series, across a capacitor.
 20. An antenna comprising: a first, outer conductor; at least one inner conductor within the first outer conductor; the at least one inner conductor being inductively coupled to the first outer conductor and electrically unconnected to the first outer conductor; and a second outer conductor surrounding the first outer conductor; wherein the second outer conductor defines a plurality of holes therethrough; the at least one inner conductor comprises a plurality of inner conductors; and at least two of the plurality of inner conductors are connected to each other in parallel, across a capacitor.
 21. An antenna comprising: a first, outer conductor; at least one inner conductor within the first outer conductor; the at least one inner conductor being inductively coupled to the first outer conductor and electrically unconnected to the first outer conductor; and a second outer conductor surrounding the first outer conductor; wherein the second outer conductor defines a plurality of holes therethrough; the antenna is a receiving antenna; the second outer conductor is configured to receive electromagnetic radiation; the first outer conductor is configured to receive some electromagnetic radiation directly, though the plurality of holes; and the second outer conductor is electrically unconnected to the first outer conductor and the second outer conductor is inductively coupled to the first inner conductor. 